As is known, use of LED (Light Emitting Diode) light sources is increasingly growing in various fields of application (e.g. automotive, home automation, consumer and industrial electronics, etc.), due to their high efficiency and low consumption.
In particular, LEDs are nowadays widely used in avionics applications, as light sources in cockpit lighting systems. LEDs are used in a plurality of equipments inside the cockpit, for example as backlight for displays, in warnings or advisory annunciators lighted panels, lighted control keys, etc. By means of dedicated actuators (e.g. switches or potentiometers) arranged inside the cockpit, personnel of the crew may adjust brightness of the light sources, and select a lighting mode, e.g. a ‘BRIGHT mode’ corresponding to a maximum brightness for daylight sun condition, a ‘DIM mode’ corresponding to a minimum brightness for night condition, or an ‘NVG mode’ corresponding to a brightness value suitable for use with night vision goggles. In a known architecture, commands imparted by the user are received by a control unit of the lighting system (generally known as “Dimming Control Unit” or DMCU), which is configured to process the information received, and to generate control and/or driving signals (in general, management signals) necessary to manage the light sources of the various equipments in the cockpit.
Pulse width modulation has proven to be a reliable solution for varying the intensity of the light emitted by LED light sources gradually (operation commonly known as “dimming”), and envisages the use of square wave signals having a variable duty cycle. The light emitted by a LED is a substantially linear function of the duty cycle of the PWM driving waveform, and also shows a non-linear dependency on the amplitude of the same waveform; dimming can thus be achieved by variation of either the duty cycle or the amplitude of the PWM signal, or both. However, due to the fact that typically a uniform spectrum of emission is required and the amplitude of the driving signal influences the colour of the emitted light, and that noise and environmental disturbances may easily affect the amplitude information, it is commonly preferred to achieve dimming by using PWM square wave signals having fixed amplitude and variable duty cycle.
Management signals transmitted from the DMCU to the various equipments and corresponding light sources may include: PWM control signals, i.e. PWM signals (usually voltage signals) having a low or very low current capability, and a duty cycle that is adjusted based on the desired brightness level (i.e. the duty cycle “codes” the brightness information); discrete control signals (usually logic signals having two discrete logic values), that are associated to the control signals and carry control information such as the desired lighting mode (BRIGHT, DIM, NVG), or test information (TEST); and PWM driving signals (either voltage or current signal), i.e. PWM signals with low, medium or high power capability adapted to directly drive a LED light source according to the desired brightness and lighting mode, and thus having suitable amplitude and duty cycle values. Each equipment is provided with a decoding interface, adapted to receive the PWM and discrete control signals in order to decode duty cycle and amplitude information therefrom; and with an internal driver, adapted to generate a PWM driving signal to drive the internal light source (or light sources in the event that the equipment is provided with a plurality of light sources) based on the decoded information. The decoding interface and internal driver are bypassed, if PWM driving signals are exchanged between the DMCU and the equipment (the PWM driving signal energy is used to directly drive the light sources). In particular, the decoding interface may include a memory, and the PWM and discrete control signals define the address of a look-up table where the values of duty cycle and amplitude for driving the LED light sources are stored. The internal driver supplies the LED light sources with a controlled current, either generating a current waveform, or generating a voltage waveform through a resistor (normally present on the load side).
The requirements of a cockpit lighting system are very stringent about optical performances. In particular, it is necessary to provide: high brightness dynamic to guarantee optimal visibility in very different light conditions (e.g. in sun condition and during night flight with night vision goggles, in military applications); accurate optical spectrum control to guarantee stable and correct colour and reduced emitted energy (radiance); and high light uniformity among different regions inside the cockpit, and among the different equipments and light sources. Moreover, avionics lighting system should comply with the general requirements of avionics applications, among which: weight control and reduction; power loss reduction and current consumption limitation; maintainability and easy testability; flexibility and reliability; and compliance with EMC constraints (in terms of emission and susceptibility).
In particular, radio frequency (RF) immunity is a very critical parameter for avionics applications. Equipments must pass severe susceptibility tests, in the presence of high frequency energy injection on the equipment cables (conducted susceptibility), or high energy radiated field (radiated susceptibility). These immunity requirements are mainly due to the field of application; the extended glass surface allowing a very high radiated field directly inside the cockpit when the aircraft or helicopter is lighted by an external radar; the number of equipments inside the cockpit that can radiate energy and cause functional problems on other units with low susceptibility threshold level; and the coupling with interconnection cables and power lines.
It is clear that all the above requirements make the design of an avionics lighting system a delicate and complex task. The use of LED light sources and PWM control has allowed to meet most of the above requirements. However, some problems still limit the potentiality of avionics LED lighting systems, among which are those related to system complexity in terms of the total amount of wirings and signaling between the DMCU and cockpit equipments.
In particular, current solutions to limit RF emissions envisage the use of either shielding cables or of twisted (or balanced, or symmetric) pairs to carry signals from the DMCU to the various equipments, to the detriment of cabling complexity, weight and manufacturing costs.
Generation of the driving signals for the light sources internally to the various equipments may improve robustness against RF disturbances and electromagnetic interference, since control signals transmitted from the DMCU may have parameters optimized to comply with EMC requirements. However, this kind of solution requires a huge amount of cabling between the DMCU and the equipments of the lighting system, again to the detriment of system complexity, weight and manufacturing costs. In particular, at least one PWM control signal (to control the brightness level with duty cycle coding) and a number of discrete control signals (each corresponding to a desired lighting mode) are to be exchanged between the DMCU and the various types of light sources inside each equipment. Considering that inside the cockpit it is common to have hundreds of light sources (belonging in groups to different equipments), it is clear that wiring complexity may become an important issue.
The above wiring complexity problem is even more evident if the possibility to test the functionality of the various equipments is to be provided (as it is required in the majority of avionics applications). In this case, at least a further discrete control signal for each light source must be provided to allow an exchange of status information with the DMCU.